The present invention relates to mobile communications systems such as cellular telephone systems, and, more specifically, to a system for reducing power consumption in a portable telephone of such a system.
A cellular telephone call can be generally described as having two modes: talk and standby. A mobile station in talk mode actively exchanges data on a traffic channel, such as that for a voice or data call. In between calls, a mobile station remains in standby mode, where it must monitor a paging channel to receive messages from a base station, such as the receipt of a page necessary to initiate a call.
Since only a small fraction of the messages on the paging channel are addressed to any particular mobile station, a mobile station can reduce its power consumption, and hence increase its standby time and battery life, by periodically rather than continuously monitoring a channel for incoming messages. Such a system, often referred to in the art as a slotted paging channel, has been described in U.S. Pat. No. 5,392,287 entitled xe2x80x9cAPPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE RECEIVERxe2x80x9d, issued Feb. 21, 1995, assigned to the assignee of the present invention and incorporated herein by reference.
In the just-mentioned patent, a communication system is described having a fixed transmitter and one or more mobile stations, in which periodic messages from the transmitter to the mobile stations are scheduled in time xe2x80x9cslots.xe2x80x9d Each mobile station is assigned a time slot during which it monitors transmissions. The transmitter transmits messages to a particular mobile station only during the slot assigned to that mobile station. The mobile station 30, goes into an xe2x80x9cactive statexe2x80x9d during its assigned slot. It may remain in the active state after its assigned slot if the message requires the receiver to perform additional actions. This active state is often referred to as the xe2x80x9cawakexe2x80x9d state.
Messages transmitted on the paging channel from the transmitter to the mobile station may include those for alerting the mobile station to the presence of an incoming call (i.e. a xe2x80x9cpagexe2x80x9d messagexe2x80x9d) and those for periodically updating system parameters in the mobile receiver (i.e. xe2x80x9coverheadxe2x80x9d messages). A list of exemplary messages found on a typical paging channel may be found in Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95, entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d at section 7.7.2.1. Each of the messages described in this section may contain a field that indicates whether another message is forthcoming. If a mobile station receives a message indicating that an additional message is forthcoming, the mobile station will remain in the awake state to receive additional messages. Once there are no additional messages, the mobile station may immediately enter an inactive state. During the time period between successive occurrences of its assigned slot, the mobile station may conserve power during this time by shutting off power and/or clocks to one or more components. For example, the components used for monitoring the transmissions are not needed outside a mobile station""s assigned slot since no data for that mobile station will be sent. This inactive state is often referred to as a xe2x80x9csleepxe2x80x9d state.
When a mobile station is in standby mode, the average current consumed across a slot determines the amount of standby time that can be achieved with a given battery size. The average current consumed across a slot equals the awake current weighted by the fraction of time spent in the awake phase of the slot, summed with the sleep current weighted by the fraction of time spent in the sleep phase of the slot.
For a given battery size, standby time can be improved by reducing the average current consumed across a slot. This can be achieved by either reducing the current consumed in the awake phase, reducing the current consumed in the sleep phase, or by spending less time in the awake phase and more time in the sleep phase. Typically the fraction of time spent in the awake phase is a much smaller interval than the fraction of time in the sleep phase, but since the active phase current is generally many times greater than sleep phase current, any reduction in the amount of time spent in the awake phase can result in a direct and significant improvement in standby time.
In order for the message to be decoded with high reliability, the awake time must include the time needed to reinitialize the analog receiver chain to provide valid receive samples, the time needed to search these samples for multipath and assign fingers to provide a valid symbol stream, and the time needed to initialize the state metrics with the symbol stream before the symbols associated with the message of interest. During the initialization of the analog receiver chain, the frequency synthesizer used to mix the received signal down to baseband frequency must come into lock, and gain scaling and DC bias loops, if any, must lock to provide a valid baseband receive sample stream. The paging channel used in IS-95 is continuously encoded with a K=9 rate xc2xd convolutional code. Several constraint lengths of symbols must be provided to the Viterbi decoder to initialize its state metric values in advance of the symbols of interest.
To demodulate the paging channel, the mobile needs to acquire a precise timing reference for each of the multipath components that comprise the received waveform. Demodulator fingers, each with their own specific time reference, are assigned to the individual multipath components. Each finger despreads the receive samples at its assigned time reference. The despread results from each finger are then combined to form a single symbol stream for decoding. Such a demodulator, known as a RAKE receiver, is described in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, issued Apr. 28, 1992, assigned to the assignee of the present invention and incorporated herein by reference.
During initial power-on acquisition, all possible shifts of the PN sequences are searched to acquire the timing references for each finger. Searching each PN offset can take anywhere from hundreds of milliseconds to a few seconds depending on the channel conditions during acquisition. Performing such a full reacquisition at the start of every slot in standby mode would take too long and consume too much standby current for a practically sized portable phone battery. Instead, the clock to the demodulator circuit is gated off for a precise duration of time which is designed so that the circuits are automatically aligned with the system when their clocks are gated back on. The time period is measured by a sleep timer clocked off a high precision oscillator. During the sleep interval, only the oscillator and the sleep timer are active.
In IS-95 systems, the interleaver in the transmitter and the deinterleaver in the mobile station process data on 20 ms frame boundaries, and both must be aligned with each other. The PN generators in the transmitter and the mobile station have sequence lengths of 215. Each PN generator updates at a rate of 1.2288 MHz and so the PN sequence period is 26.66 ms. The PN generators in the mobile station must be aligned with the respective PN generators in the transmitter. The smallest period that is common to both the interleaver/deinterleaver timing and the PN sequence timing is 80 ms. Exactly 3 PN sequence periods of 26.66 ms and exactly 4 interleaver frames of 20 ms fit into an 80 ms period. More generally, the sleep interval is programmed in steps of the least common multiple of the two intervals.
The sleep timer is programmed to sleep for a multiple of the 80 ms period to ensure that both the timing reference of the fingers and the frame reference timing of the deinterleaver do not change with respect to actual time, or xe2x80x9cwall clockxe2x80x9d time. If the sleep timer is programmed with a value other than a multiple of 80 ms, when the demodulator wakes up the PN generators and/or the interleaver timing will not be aligned with the proper system time and demodulation will be impossible.
After the sleep timer expires, the demodulator circuits are re-enabled. The demodulator circuits are aligned with the transmitter, and the fingers are demodulating at the offsets that were optimal for the multipath environment that was present prior to the previous sleep. Since the multipath environment will probably have changed during the sleep interval, the optimal finger assignments can be reacquired with a short search of PN offsets centered around the multipath that was present in the previous slot cycle, avoiding the need to search through all possible PN offsets of the PN sequence.
The present invention is an improved method and circuit for reducing the amount of time a mobile station spends in the active phase of a slot in a communications system utilizing a slotted paging mechanism. Instead of an awake time quantized to increments of the least common multiple of the PN sequence period and the deinterleaver frame duration, the invention allows the awake time to be configured for the minimum interval needed to initialize the analog receiver chain, reacquire the finger time references, initialize decoder state metrics, and demodulate the paging channel messages of interest.
The invention adjusts the timing reference of the fingers to compensate for a sleep duration not an integral multiple of the PN sequence period, and adjusts the deinterleaver frame reference timing to compensate for a sleep duration not an integral multiple of the frame interval. In addition, the invention initializes the frequency tracking, gain scaling, and DC bias loops, if any, of the analog receiver chain to minimize the time needed to relock these loops.